Fluid working machines are generally used, when fluids are to be pumped or fluids are used to drive the fluid working machine in a motoring mode. The word “fluid” can relate to both gases and liquids. Of course, fluid can even relate to a mixture of gas and liquid and furthermore to a supercritical fluid, where no distinction between gas and liquid can be made anymore.
Very often, such fluid working machines are used, if the pressure level of a fluid has to be increased. For example, such a fluid working machine could be an air compressor or a hydraulic pump.
Generally, fluid working machines comprise one or more working chambers of a cyclically changing volume. Usually, for each cyclically changing volume, there is provided a fluid inlet valve and a fluid outlet valve.
Traditionally, the fluid inlet valves and the fluid outlet valves are passive valves. When the volume of a certain working chamber increases, its fluid inlet valve opens, while its fluid outlet valve closes, due to the pressure differences, caused by the volume increase of the working chamber. During the phase, in which the volume of the working chamber decreases again, the fluid inlet valve closes, while the fluid outlet valve opens due to the changed pressure differences.
A relatively new and promising approach for improving fluid working machines are the so-called synthetically commutated hydraulic pumps, also known as digital displacement pumps or as variable displacement pumps. Such synthetically commutated hydraulic pumps are known, for example, from EP 0494236 B1 or WO 91/05163 A1. In these pumps, the passive inlet valves are replaced by electrically actuated inlet valves. Preferably the passive fluid outlet valves are also replaced by electrically actuated outlet valves. By appropriately controlling the valves, a full-stroke pumping mode, an empty-cycle mode (idle mode) and a part-stroke pumping mode can be achieved. Furthermore, if inlet and outlet valves are electrically actuated, the pump can be used as a hydraulic motor as well. If the pump is run as a hydraulic motor, full stroke motoring and part-stroke motoring is possible as well.
A major advantage of such synthetically commutated hydraulic pumps is their higher efficiency, as compared to traditional hydraulic pumps. Furthermore, because the valves are electrically actuated, the output characteristics of a synthetically commutated hydraulic pump can be changed very quickly.
For adapting the fluid flow output of a synthetically commutated hydraulic pump according to a given demand, several approaches are known in the state of the art.
It is possible to switch the synthetically commutated hydraulic pump to a full-stroke pumping mode for a certain time, for example. When the synthetically commutated pump runs in a pumping mode, a high pressure fluid reservoir is filled with fluid. Once a certain pressure level is reached, the synthetically commutated pump is switched to an idle mode and the fluid flow demand is supplied by the high pressure fluid reservoir. As soon as the high pressure fluid reservoir reaches a certain lower threshold level, the synthetically commutated hydraulic pump is switched on again.
This approach, however, necessitates a relatively large high pressure fluid reservoir. Such a high pressure fluid reservoir is expensive, occupies a large volume and is quite heavy. Furthermore, a certain variation in the output pressure will occur.
So far, the most advanced proposal for adapting the output fluid flow of a synthetically commutated hydraulic pump according to a given demand is described in EP 1 537 333 B1. Here, it is proposed to use a combination of an idle mode, a part-stroke pumping mode and a full-stroke pumping mode. In the idle mode, no fluid is pumped by the respective working chambers to the high-pressure manifold. In the full-stroke mode, all of the usable volume of the working chamber is used for pumping fluid to the high-pressure side within the respective cycle. In the part stroke mode, only a part of the usable volume is used for pumping fluid to the high-pressure side in the respective cycle. The different modes are distributed among several chambers and/or among several successive cycles in a way, that the time averaged effective flow rate of fluid through the machine satisfies a given demand.
The controlling methods, which have been employed so far, had in common, that the control algorithm did the necessary calculations “online”, i.e. during the actual use of the fluid working machine. For this, a variable, the so-called “accumulator” was used. The accumulator uses the fluid flow demand as the (main) input variable.
During the use of the fluid working machine, the value of the accumulator is checked and it is determined, whether a pumping stroke should be initiated, or not. In the next step, the accumulator is updated by adding the actual fluid flow demand. Furthermore, an appropriate value is subtracted from the accumulator, if some pumping work has been performed. Then, the loop is closed.
While these “online” controlling methods are relatively easy to implement, especially the controlling methods which are publicly known so far, they still suffer from certain limitations and draw-backs. A major issue is, that the time responsiveness, i.e., the time, the fluid working machine needs after a change in fluid flow demand to adjust its fluid flow output, can be quite long, especially under certain working conditions. Furthermore, under certain working conditions, huge variations in the output characteristics of the fluid working machine, and therefore strong pressure pulsations on the high-pressure side can be observed. Such pressure pulsations can be noticed in the behaviour of a hydraulic consumer (e.g. a hydraulic piston or a hydraulic motor). The pulsations can be noticed as a startstop-like movement (a “stiction” behaviour). The pressure pulsations can even lead to the destruction of certain parts of the hydraulic system.
To solve these problems, several improvements have been considered, addressing various issues. While some of these improvements are addressing some of the underlying problems quite efficiently, certain issues are still not addressed by these improvements.
A major imperfection is that when using “online-algorithms” with digital (i.e. discrete) controllers, numerical artefacts can never be completely avoided. This can be considered as some sort of a “Moiré”-effect for synthetically commutated hydraulic pumps. These numerical artefacts can occur especially when the fluid flow demand varies in a continuous way over time. In fact, quite often strong fluctuations in fluid flow output and even gaps, in which no pumping is performed at all for an extended period of time, can be observed when employing previously known “online” control algorithms.